Towards commercialising cypress as a high value plantation forest species
Dean Satchell, New Zealand Tree Grower August 2017.
Cypress species are well known in New Zealand for their timber, usually called macrocarpa, this being a namesake for the species Cupressus macrocarpa. Macrocarpa and hybrids have been planted for shelter, shade and aesthetic values for well over 100 years, primarily because of easy establishment and good growth throughout New Zealand. Older plantation and shelterbelt trees have often proved to be very robust, growing into large trees free from disease, standing the test of time to eventually be the target of loggers and sawmills satisfying demand for the timber. The species achieved recognition in New Zealand because of the quality of the timber, which has a rich colour and lustre similar to kauri, a traditional native appearance timber now in short supply.
Price and demand for quality cypress timber remains high, but problems with form, slow growth and disease levels that have resulted from growing unselected seedlots has progressively diminished grower confidence and interest in the species. Poor volumes of poor quality logs mean little value to the grower. Overcoming these problems by making high quality clones available to growers could transform the economics of cypress forestry.
For cypress to be a viable commercial forestry crop, trees need to be available which grow fast, grow straight, produce a high proportion of heartwood and be resistant to disease. All of these need to be rolled into one tree, but also consistent between trees in the stand. Clonal forestry potentially offers all of the above via a single selection process.
One advantage with cypress over other commercial plantation conifers grown in New Zealand is that clones can be selected which apparently do not suffer from physiological ageing. This means that continued propagation does not result in slower growth of propagules. The opportunity is for one-off selection of superior clones which can continually be reproduced as cuttings. Clonal selection also offers the ability to test and select for desirable wood properties such as colour, heartwood content, durability level, density and strength.
However, the length of time a clone should be tested in field trials before it can be recommended for use is not well understood and is open for question. Early selection comes at a lower cost but entails greater risks once planted. This is because some poor traits may not be expressed in immature trees. How long does it take for cypress to express undesirable traits?
This research evaluated during late 2016 involved over 2,500 trees from 11 clonal trials planted between 1994 and 1998. The primary purpose of the research was not to evaluate performance with the aim of making selections for deployment, but rather to evaluate relative performance and health for clones between different sites and geographical regions. Also of interest was identifying issues relevant to selection of clonal cypress tree stock.
Performance of the clones was assessed using a rating method and four performance criteria − tree size, tree form, stem health and foliar health. Of interest to growers are three key performance and siting parameters relevant to clonal selections −
- Overall performance of each clone
- Each clone’s relative adaptability to different sites
- How each site performed in general.
By evaluating each clone over a range of sites, its adaptability to different sites can be assessed as relative performance between sites. The relative performance of all clones between sites can also be assessed. Furthermore, by evaluating each clone against the other clones across all sites, differences between clones and differences between sites are revealed.
Of particular interest was within-clone variability, across sites and within site. A good clone is consistently good. A clone that performs well in one site but not another should not be a candidate and would not be selected if properly tested across a range of sites.
Each site has different problems such as level of wind, soil nutrition, soil drainage, soil moisture, rainfall, humidity and temperature, along with seasonal variations. Each of these different variables might affect all clones or individual clones in different ways. Climatic explanatory variables used in this research were seasonal rainfall and seasonal temperature.
Although most of the clones in these trials performed very poorly, this result supported the reasoning that comparing relative performance between clones and sites was the intent for this research. Mediocre clones offer insight − hostile sites prove more challenging for mediocre clones than good clones and so the data reveals the challenging variables.
Both growth and form as well as stem and foliar disease were found to be strongly influenced by site. That is, relative performance of different clones differs significantly by site. Some clones proved to be more resilient than others to site or climatic factors. For example Ovensii proved to be healthy at all sites but growth and form was variable, especially in exposed South Island sites. Ferndown, on the other hand, had variable health on all sites and on average was healthy in cooler Bay of Plenty sites but not Northland, while growth and form scores were more variable than for Ovensii.
Dry or cool conditions alone were insufficient to significantly reduce disease levels, but cool dry conditions consistently produced low disease levels. The old adage that you can avoid canker by growing cypress in the South Island can be dismissed. Canker was rife in some South Island locations but virtually absent in the central high country.
Some clones did perform well throughout all locations they were present. This illustrates the commercial potential for the species if high quality resilient clones were used in production forestry.
Very few if any of the clones evaluated from these historical trials are suitable as the one silver bullet for growers. The clones were mostly selected on early growth and form and some were selected from single trees based on their performance in one site. We now have insight and can conclude that early selection must be avoided.
We can also conclude that selections could still be made from single trees, provided the propagules were thoroughly tested before considering for deployment. The two BHYB (Barr Hybrid) clones showed promise by growing into very good trees across a range of sites showing resilience, whereas other clones such as PS did sometimes perform well, but in few sites, so were not resilient. Clonal trials over a wide range of sites reveal the very best, most resilient clones. Identifying such adaptability must precede use. The most promising clones in these trials are the two Neil Barr hybrids BHYB1 and BHYB2, but unfortunately these were not planted across the full range of sites.
A key result of this research is that evaluating performance of clones for selection requires sufficient tree maturity for adequate expression of traits, both good and bad. A good-looking lightly branched young tree may become heavily branched and fluted on maturity, such as the NZR clone. In addition to growth, form and disease resistance, selection criteria recommended for clonal selection programmes includes light branch structure at maturity, low levels of fluting, a low incidence of stem breakages, resistance to toppling and possibly even resilience to low nutrient status in soil. Based on observations undertaken in the course of this research, in the author’s opinion, the length of time required for trialing clones across a wide range of sites before claiming resilience should be no less than a decade and ideally would be 15 years.
Most importantly, what these historical trials do are to offer lessons for applying to future selection and trial work and research questions which could form the direction for future work. All too often, we look forward without looking back, but with the time frames involved in growing trees, any insight from what was done some time ago has got to be a good thing.